Spring wind events as drivers of lake stratification anomalies: a 1D modelling case study of Lake Erken

Sammanfattning

In August 2025, Stockholm Water and Waste issued water use restrictions after warmer than expected bottom water temperatures in Lake Mälaren reduced the city's drinking water production capacity. The same regional meteorological conditions affected Lake Erken, a lake in east-central Sweden, which exhibited anomalously weak thermal stratification during the summers of 2019 and 2025. This study used the one-dimensional General Ocean Turbulence Model (GOTM) combined with meteorological scenario analysis to attribute these anomalies to specific drivers and to project how often similar events may occur under future climate scenarios. Spring wind speed was identified as the dominant driver of the suppressed stratification, with cloud cover, shortwave radiation and precipitation having negligible effects and air temperature producing only a secondary contribution. The two anomalous years illustrate two pathways to the same outcome, with 2019 driven by a small number of well-timed wind events at the onset of stratification and 2025 by frequent events distributed throughout the April-June spring window. Future projections under SSP1-2.6, SSP3-7.0 and SSP5-8.5 showed that years with weak stratification concentrate in the 2020s and 2030s under all three scenarios, identifying the near-term future as the period of highest occurrence regardless of emission pathway and that under SSP3-7.0 and SSP5-8.5 the weak stratification become less frequent. Beyond the 2030s the scenarios diverge, under SSP3-7.0 and SSP5-8.5 the warming thermal background progressively overrides the wind suppression mechanism and analogue years decline toward zero by the end of the century, while under SSP1-2.6 analogue years persist throughout the century.